Glutamate may play an important role in integrating the metabolic activity of the placenta and the fetal liver. Studies in fetal lambs have demonstrated a large net flow of glutamate from the fetus to the placenta and in opposite direction to a large net flow of glutamine from placenta to fetus. The fetal liver shows the reverse, i.e., net glutamate output and net glutamine uptake. To clarify the physiological meaning of these observations, experiments will be performed in fetal lambs under normal physiologic conditions using both stable isotope tracers and 14C labelled glutamate. The tracers will be infused in either the fetus, the mother or both for the purpose of measuring (1) the disposal rate of fetal plasma glutamate, its uptake by the placenta, its decarboxylation rate and conversion to glutamine in both fetus and placenta, (2) the disposal rate of fetal plasma glutamine and its conversion to fetal plasma glutamate by the whole fetus and by the fetal liver, (3) the entry rate of maternal glutamate and/or products of its placental metabolism into the fetal circulation, (4) the entry rate of maternal glutamine into the fetal plasma glutamine pool and (5) the effect of increasing the supply of fetal glutamate to the placenta on the placental uptake and metabolism of fetal glutamate, and on placental 02 consumption rate.